The space (if any) between the keyword and form is not stripped
when inserting the form into the autoloads file, which looks odd.

In Emacs the number of autoload cookies with and without such a
space, is almost the same, so that provides no guidance as to what
should be preferred.  Go with what makes the aligner in me happy.

- Cannot use `require' because "magit-autoloads.el" may fail to
  `provide' the feature.

- Cannot use `load' unconditionally because if "magit-autoloads.el"
  were already loaded that would load it again.

- Cannot use just `featurep' to determine whether the file has been
  loaded because "magit-autoload.el" may not `provide' the feature.

Closes #5466.